Superconductivity is STILL a quantum mechanical phenomenon. It is characterized by the Meissner effect, the complete ejection of magnetic field lines from the interior of the superconductor as it transitions into the superconducting state. The occurrence of the Meissner effect indicates that superconductivity cannot be understood simply as the idealization of perfect conductivity in classical physics. YBa2Cu3O7, one of the first cuprate superconductors to be discovered, has a critical temperature of 92 K, and mercury-based cuprates have been found with critical temperatures in excess of 130 K. The explanation for these high critical temperatures remains UNKNOWN! The resonating valence bond (RVB) theory states that in "copper oxide lattices, electrons from neighboring copper atoms interact to form a valence bonds." We should take into consideration the effects of high pressure. H2S has been observed to exhibit superconductivity at below 203 K but at extremely high pressure around 150 gigapascals. My guess is that complex orbital (p,d,f) overlap could help to explain superconductivity. What is the influence or changes within the complex orbital (p,d,f) overlap of a superconductive materials as it transitions into the superconducting state?
Dear David,
The high critical temperature superconductors have some features different of the BCS metallic compounds as mercury. One of the important, with respect to your question, is that they have an isotope effect is relatively weak with respect to metallic ones. This seems to imply that the electron-phonon interaction is less important that the electron-electron. The main idea behind this electron-electron is that there are some spin configurations which have attractive interaction between them and therefore could allow to explain their bosonization in pairs close to the Fermi level, substituting in some aspects the phonon of the BCS. Thus here it is very important if the electrons are s, p, d or f because their associated magnetic configurations are very different due to the exchange interactions on their electronic spins.
I hope this helps.
Dear David,
But your last remark or question:
What is the influence or changes within the complex orbital (p,d,f) overlap of a superconductive materials as it transitions into the superconducting state?
is very difficult to say because there is not a model that can take this overlapping into account. What seems quite logical is that this relationship would exists if magnetism is going to be the responsable of the interaction between electrons close to the Fermi level (Frölich mechanism) instead of the phonons. For instance, the 10 d-electrons can be classified within eg or t2g energy levels with very different wave symmetries, which makes them quite different magnetic behaviour.
Hi David,
The superconductivity of H2S is well understood by the conventional BCS theory. To have such high Tc one need high energy phonon mode (that is why people are searching in light atoms, which gives high energy phonon mode) and high density of state (that is why we need high pressure).
For unconventional superconductivity, cuprate, iron-based superconductors et at, the microscopic mechanism is different, even though we don't know exactly what it is. The orbital over lapping is certainly important, as it gives you kinetic energy which is essential in strongly correlated materials.
Thank your for your answers. The electron-phonon interaction is less important at absolute zero. At absolute zero temperature, a crystal lattice lies in its ground state, and contains no phonons. At the higher temperature the phonon energy (modes) are greater. It could be stated with further research that phonons are a result but not the source of superconductivity.
For example in the paper Low temperature x-ray diffraction study on superconductivity (Y.Zue) a clear lattice kink can be observed in the d values in the vicinity of Tc. There is a change in the phonon and static atomic positions within the cell. At present, the origin of this marked change of the lattice remains unknown.
What causes of the lattice kink at Tc? Are the complex orbitals constant or do they change with temperature or pressure?
Dear David,
The key idea is not the phonon vibration itself, but its coupling with electrons. A heuristic picture is that one electron couple to its surrounding atoms and generate local atomic deformation. Since electron moving much faster than atoms, after this electron left, the atomic distortion still there and attract another electron.
Electron-phonon coupling is very sensitive to the density of state near EF (Fermi energy). In the superconducting state, the Fermi surface is gaped which will have a feedback effect on the phonons.
As I said before, you will always need orbital overlapping, otherwise it is not a many body system. One thing we should keep in mind is that the orbital overlapping has an energy scale of electron volt, while the superconducting pairing has an energy scale of mili-electron volt (10^-3 eV).
There are two parts to this question to consider what is the CAUSE and what is an EFFECT. Many researcher are searching for lattice anomalies at the superconducting temperature.
The theory is well known that the behavior of superconductors suggests that electron pairs are coupling over a range of hundreds of nanometers, three orders of magnitude larger than the lattice spacing. Called Cooper pairs, these coupled electrons can take the character of a boson and condense into the ground state. An electron pair or a Lewis pair consists of two electrons that occupy the same orbital but have opposite spins.
In solid matter the density of states (DOS) of a system describes the number of states per interval of energy at each energy level that are available to be occupied.
My question is "What happens to the DOS when the complex orbitals overlap in solid materials?" There is not a model that takes this overlapping into account which spreads over a large lattice spacing.
You do not have to answer this question based other researchers existing conclusions. I can Google those. I would like to know what your own creative thoughts and theories about the CAUSES for superconductivity using known results.
Hi David,
I was trying to give a very general argument on the role of orbital overlapping and how it can affect superconductivity. I do not prefer to mix the physics of unconventional superconductivity (e.g.YBCO, BaKFe2As2 et al) and conventional superconductivity as the latter case does not apparently involve the antiferromagnetic correlations (J*S_i*S_j term).
Back to your question, my understanding to the role of orbital overlapping: 1. it affects the super-exchange interaction J. 2. It will tune the band width and the electron filling. 3. It can change the screening of the coulomb interaction. In order to optimise the high-Tc one really need to fine tune these parameters, because, superconductivity is not the only symmetry breaking state, CDW, SDM and other orders may also appear in the parameter space.
While I agree with your general consideration, but exactly how it drives superconductivity will need more works.
Hopefully, my answer helps.
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